When a veterinarian performs a lameness examination, he or she often will use nerve blocks to try and determine the location of the problem. The areas are “blocked” so that they become numb to pain, revealing which structures are involved in causing lameness.
In the equine lower limb, there are two distinct sets of peripheral nerves: sensory (afferent) nerves and motor (efferent) nerves. The motor nerves conduct information from the brain and spinal cord to the limbs and cause the muscles to move the limbs. The sensory nerves conduct information like pain, pressure, and temperature from the limbs to the spinal cord and brain. Sensory nerves originate in the hoof and gather information as they ascend toward the spinal cord (Figure 1). They also conduct information from areas on the limb that are clearly described and very repeatable from horse to horse. With this in mind, equine veterinarians are able to perform diagnostic nerve blocks with local anesthetics.
Analgesia of these nerves can selectively eliminate or “block” portions of the limb from sending sensations of pain to the brain. A horse with a lameness is considered to have “blocked sound” when the lameness that was seen originally is eliminated by local anesthesia. Using this principle, veterinarians are able to identify the specific location on the limb responsible for the horse’s pain and subsequent lameness. Therapies can be directed to exact locations and lamenesses can be resolved.
Local anesthetics are drugs that block the nerves’ ability to conduct a nerve impulse. In addition, these anesthetics are effective only at the specific location where they were deposited, leaving the proximal aspect of the nerve to function normally. Therefore, they interfere with the sensory nerves’ ability to transmit information that a given area is painful (analgesia).